Monsoonal Damping Effect and Wake Effect on the Diurnal Cycle of Winds and Rainfall Variability over the Maritime Continent

Two mechanisms associated with multi-scale climate processes of ENSO, monsoon and diurnal cycle are found important for rainfall variability over the Maritime Continent: 1) Monsoonal damping effect over narrow islands such as Java and North Borneo - an inverse relationship between the monsoonal wind speed and the intensity of diurnal cycle of land-sea and mountain-valley winds; 2) Wake effect over wide islands such as South Borneo - the diurnal cycle is stronger on the wake- or lee-side than the windward side of the island or mountain ranges in respect to the low-level monsoonal winds. The mechanisms for the spatial heterogeneity of climate variability over Java Island have been studied. Besides the well-known anomalous dry conditions that characterize the dry and transition seasons during an El Niño year, analysis of regional model output reveals a wet mountainous south versus dry northern plains in precipitation anomalies associated with El Niño over Java during the peak rainy season. Modeling experiments indicate that this mountains/plains contrast is caused by the interaction of the El Niño–induced monsoonal wind anomalies and the island/mountain-induced local diurnal cycle of winds and precipitation. During the wet season of El Niño years, anomalous southeasterly winds over the Indonesian region oppose the climatological northwesterly monsoon, thus reducing the strength of the monsoon winds over Java. This weakening is found to amplify the local diurnal cycle of land–sea breezes and mountain–valley winds, producing more rainfall over the mountains, which are located closer to the southern coast than to the northern coast. The interannual variability of precipitation over Borneo Island in association with ENSO has been studied by using the satellite and reanalysis data. Analysis of the GPCC precipitation shows a dipolar structure of wet southwest versus dry central and northeast in precipitation anomalies associated with El Niño over Borneo Island during the austral summer. The spatial distribution of rainfall over Borneo depends on the direction of monsoonal winds. Weather typing analysis indicates that the dipolar structure of rainfall anomalies associated with ENSO is caused by the variability in the frequency of occurrence of different weather types. Rainfall is enhanced in the coastal region where sea breezes head against off-shore synoptic-scale low-level winds, i.e., in the lee side or wake area of the island, which is referred to here as the "wake effect." In the December-February of El Niño years, the northwesterly austral summer monsoon in South Borneo is weaker than normal over the Maritime Continent and easterly winds are more frequent than normal over Borneo, acting to enhance rainfall over the southwest coast of the island. This coastal rainfall generation mechanism in different weather types explains the dipole pattern of a wet southwest versus dry northeast in the rainfall anomalies over Borneo Island in the El Niño years.